The present invention is in the field of turbomachinery and, more particularly, centrifugal compressors.
In the field of turbomachinery, some compressor applications such as automotive turbochargers and aircraft air conditioning systems require wide stable operating flow range. Flow range in centrifugal compressors is bounded by surge conditions and choke conditions. Surge conditions develop when volume flow is too low and a discharge process is interrupted. Under surge conditions undesirable cyclical flow instability develops. Choke conditions develop when gas flow at some point in the compressor reaches sonic velocity. Under choke conditions, decreasing compressor outlet pressure does not produce increased mass flow for a given speed and inlet conditions. Thus, mass flow through a compressor is bounded by a minimum associated with surge conditions and a maximum associated with choke conditions.
Many interrelated design parameters may be used to produce a design for a compressor with desired properties. Often such designs are produced through computer generated simulations. However, as is often the case with designs having interrelated parameters, changing one design variable may adversely affect some other aspect of a compressor. Thus a compressor design for a particular application typically requires expensive iterative design trials and modeling.
After an optimized design is complete for a particular application it may be desirable to extend that design for other wider-flow range applications. Obviously, it would be advantageous to provide for a simple widening of flow range without performing a re-design. Consequently, compressor designers often seek expedients for widening a flow range of a compressor without incurring the expense of redesigning the compressor
One of the most frequently used methods to widen this flow range is to add slots or ports to a stationary shroud near a leading edge of a compressor wheel. When the compressor operates at “near surge” conditions, the ports are expected to recycle gas flow from the shroud to a compressor inlet and thus reduce the potential for surging at low flowrates. When the compressor operates at “near choke” conditions, the ports are expected to suck the flow from the compressor inlet to the shroud and thus allow increased mass flow before choke conditions develop. This ported shroud concept works well for compressors with vaneless diffusers because compressor rotating stall, which induces surging, often occurs first on the inducer, the axial portion of a wheel upstream from a diffuser. Similarly, choke conditions typically develop on the wheel in compressors with vaneless diffusers.
However, many compressor applications require a vaned diffuser because such a compressor is more efficient than one with a vaneless diffuser. In compressors with vaned diffusers, the flow instability and choke could occur first inside the diffuser (i.e., downstream of the wheel). Using a ported shroud to recycle the flow near the leading edge of a wheel of a compressor with a vaned diffuser may not improve its flow range. To the contrary, a ported shroud may actually diminish the compressor's performance.
An example of a prior art compressor with a vaned diffuser is shown in FIG. 1. In FIG. 1 there is shown a partial cross-sectional view of a compressor designated generally by the numeral 10. The compressor may comprise a wheel 12, a diffuser 14 and a housing 18. In operation, the wheel 12 may rotate and draw a gas such as air into a shroud 20. The gas may flow in a downstream direction designated by mainstream flow lines 22. As the gas passes along the wheel 12, its absolute velocity increases. The gas then traverses across the diffuser 14 where its velocity decreases and its pressure increases. The gas then passes into the housing 18 for collection.
The diffuser 14 illustrated in the compressor 10 of FIG. 1 may be provided with vanes 14a. In that regard the diffuser 14 may be referred to as a vaned diffuser. In a typical one of the compressors 10 with one of the vaned diffusers 14, a throat 24 may develop at a region of the compressor 10 between a trailing edge 12a of the wheel 12 and trailing edge 14c of the vanes 14a of the diffuser 14. The throat 24 is a region of the compressor 10 with the least flow area in which gas flow may initially reach sonic velocity. In that context, the throat may be a region of the compressor 10 in which a “choke” condition may develop. Additionally, the throat 24 is a region in which “near choke” conditions may occur.
The compressor 10 may increase the mass of gas flow across the wheel 12 as the compressor outlet pressure decreases for a given wheel rotational speed and inlet conditions. As the mass flow increases, there may be a corresponding increase in velocity of gas flow inside the compressor. When this velocity reaches Mach 1 or sonic speed, further reducing the compressor outlet pressure by opening the outlet throttle may produce no additional mass of gas flow. This phenomenon is referred to as “choke”. In a typical one of the compressors 10, the region with the least flow area of the compressor 10 at which “choke” or “near choke” conditions develop may be referred to as the throat 24.
The throat 24 may also be a region of the compressor 10 in which “surge” or “near surge” conditions initially develop. The phenomenon of “surge” may occur when the flow rate is so low that instability develops in the gas flow emerging from the wheel 12 or the diffuser 14. The phenomenon may produce cyclical surging of gas through the compressor 10 accompanied with harmful vibrational stresses
For any particular one of the compressors 10, there may be a range of mass flows that may be produced by the compressor 10. The range is bounded by the mass flow that produces choke and the mass flow that produces surge. This range is commonly referred to as “flow range”.
There has been no recognition in the prior art of a simple expedient to widen the flow range of a vaned-diffuser compressor such as the prior-art compressor 10. Typically, redesign of vaned-diffuser compressors has been required to achieve a widened flow range. As can be seen, a flow range widening expedient similar to the ported shroud of a vaneless-diffuser compressor would be desirable.